On average, 49–63% of people with intellectual disabilities and/or autism spectrum disorder (ASD) are prescribed psychotropic medications to treat psychopathology, including psychiatric illness, behaviours that challenge and the core symptoms and associated behaviours of these developmental disorders. In many cases, psychotropics, particularly antipsychotics, are used off-label without a proper indication, particularly to manage behaviours that challenge. The RCTs show moderate evidence supporting the efficacy of low-dose risperidone and some preliminary evidence for aripiprazole in treating behaviours that challenge among children with ASD and/or intellectual disabilities. The RCT-based evidence for the other psychotropics is equivocal, so no definitive conclusions can be made on their efficacy. Polypharmacy and the use of high doses of antipsychotics are prevalent in this population, leading to the risk of adverse events and drug–drug interactions. Despite various national and international guidelines, and government initiatives encouraging reduced psychotropic use, there is little evidence of this happening; on the contrary, the use of antidepressants, mood stabilisers and benzodiazepines may be increasing. A concerted multi-agency effort is urgently needed to address this significant public health concern of the overmedication of people with intellectual disabilities and/or ASD.